Visual impairment is a national and global health concern that has a negative impact on physical and mental health. The number of people with visual impairment and blindness is increasing due to an overall aging population. Visual impairment and blindness can be caused by any one of a large number of eye diseases and disorders affecting people of all ages. In one example, age-related macular degeneration (AMD) is an eye disorder that is currently the leading cause of vision loss in people fifty years of age or older in industrialized countries. It is estimated that by 2020, the number of people with AMD could exceed 196 million and by 2040, that number is expected to rise to 288 million. AMD is a degenerative eye disease that progresses from early stages to advanced stages of the disease. Risk factors for the disease include aging, lifestyle factors such as smoking, and genetics. The clearest indicator of progression to AMD is the appearance of drusen, yellow-white deposits under the retina, and it is an important component of both forms of AMD: exudative (“wet”) and non-exudative (“dry”). Wet AMD causes vision loss due to abnormal blood vessel growth in the choriocapillaris through Bruch's membrane. The most advanced form of dry AMD, known as geographic atrophy, is generally more gradual and occurs when light-sensitive cells in the macula atrophy, blurring and eliminating vision in the affected eye. While there are currently some promising treatments for wet AMD, no FDA-approved treatment exists for dry AMD or geographic atrophy.
A second example is childhood-onset Stargardt Disease (“STGD”), also known as Stargardt 1, a genetic, rare juvenile macular dystrophy generally associated with loss of central vision in the first two decades of life. STGD has a prevalence of approximately 1/20,000 affecting approximately 30,000 people in the US. STGD affects many ages, with the childhood-onset population at highest risk and most need. Patients with childhood-onset STGD tend to develop early severe visual acuity loss, significantly compromised retinal function, and rapid retinal pigment epithelial (RPE) cell atrophy with accompanying loss of retinal function. The median ages of onset and the median age at baseline examination are 8.5 (range, 3-16) and 12 years (range, 7-16), respectively. Patients with adult-onset disease are more likely to preserve visual acuity for a longer time and show slighter retinal dysfunction. STGD is an autosomal recessive genetic disease or complex heterozygous disease, caused by mutations in the ABCA4 gene. The ABCA4 gene encodes the photoreceptor protein ABCA4 Transporter, which is responsible for removal of bisretinoid fluorophores, which can include N-retinylidene-N-retinyethanolamine (A2E), all-trans-retinal and related photo-oxidation products of vitamin A aldehyde which together constitute lipofuscin from photoreceptor cells. Accumulation of all-trans-retinal in photoreceptor cells is believed to damage RPE cells via oxidative stress, and trigger or promote complement-mediated damage to RPE cells, leading to retinal atrophy. A related disease termed Stargardt-like macular dystrophy, also known as STGD3, is inherited in a dominant autosomal manner and is due to mutations in the ELOVL4 gene. ELOVL4 encodes the ELOVL4 protein, ELOVL fatty acid elongase 4. Mutations in ELOVL4 protein associated with STGD lead to mis-folding and accumulation of ELOVL4 protein aggregates in retinal cells, which impact retinal cell function, eventually leading to cell death and retinal atrophy. No treatments exist for STGD or Stargardt-like disease.